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Change in Coherence of Summer Rainfall Variability over the Western Pacific around the Early 2000s: ENSO Influence
Zhuoqi He 1,2, Weiqiang Wang, Renguang Wu, In-Sik Kang, Chao He, Xiuzhen Li, Kang Xu, Sheng Chen
1 State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China
2 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
We investigate a recent decadal modulation of interannual variability over the western Pacific around the early 2000s. Observational evidence shows that the anomalous Philippine Sea cyclonic circulation retreats eastward with the western Pacific rainfall anomaly distribution changing from a north-south tripole pattern to an east-west dipole pattern after 2003/04. These changes are attributed to a change in El Niño-Southern Oscillation (ENSO) properties and the associated Indo-Pacific sea surface temperature (SST) anomaly pattern. Before the early 2000s, slow-decaying ENSO events induce large SST anomalies in the North Indian Ocean during the following summer. The North Indian Ocean SST anomalies co-work with the opposite-sign SST anomalies in the tropical central Pacific, leading to a zonally extended anomalous lower-level cyclonic (anti-cyclonic) circulation and an elongated rainfall anomaly band over the western Pacific. After the early 2000s, ENSO events have shortened period and weakened amplitude, and the eastern Pacific SST anomalies tend to undergo a phase transition from winter to summer. Consequently, the influence of ENSO on the Indian Ocean SST anomalies is weakened and the contribution of the North Indian Ocean SST anomalies to the western Pacific summer rainfall variability becomes insignificant. In this case, the western North Pacific summer rainfall is mainly dominated by the well-developed tropical Pacific SST forcing following the early decay of ENSO events. The potential physical mechanism for the two types of ENSO influences is validated via regional de-coupled Community Earth System Model experiments.